1. Field of the Invention
This invention relates to displays which utilize a plurality of individually energizable segments to form a digit or other symbol and, more particularly, to a system and method for monitoring the fail-safe operation of such displays.
2. Background Information
Monitoring of segment type indicators is particularly important, since failure of an individual segment may result in a false indication which may not be recognizable as such. In the case of light-emitting diodes (LEDs), one solution to this problem has been to monitor the voltage across and the current flowing through the individual segments. For liquid crystal displays (LCDs), such method is not practical, since the currents to be monitored are very small.
Referring to FIGS. 1 and 2, a seven-segment LED display 1 typically comprises segments 2,3,4,5,6,7,8 which are selectively and independently driven by a display driver 10. As shown in FIG. 2, diode segments 2,3,4,5,6,7,8 are typically independently coupled between the display driver 10 and electrical ground through respective current-limiting resistors 12,13,14,15,16,17,18. Although only one seven-segment display 1 is shown in FIGS. 1 and 2, it is understood that a plurality of seven-segment displays may be arranged together in manners known in the art, thereby displaying a plurality of alphanumeric characters, symbols and/or shapes.
U.S. Pat. No. 5,703,607 discloses a drive circuit employing flip-flops and AND, OR and NOT gates for displaying seven-segment decimal digits and a partition symbol (":").
U.S. Pat. No. 5,838,290 discloses an LCD device in which an internal auxiliary voltage, which is used for control, is obtained via a photovoltaic generator or converter having a plurality of series-connected photo-sensitive diodes.
U.S. Pat. No. 5,831,693 discloses an active matrix LCD panel including a viewing area and ambient light sensors, such as photodiodes, placed on the periphery of the panel. The panel also includes a lower glass substrate and an upper glass substrate having deposited thereon filter material including red, green and blue material. A transparent conductor is deposited on the filter material to form a common plate of pixel elements. A second opposing plate is deposited on the lower glass substrate, with an individual plate deposited opposite each filter to control each red, green and blue pixel. The photodiodes output a voltage directly proportional to the ambient light intensity. The output voltage, in turn, is employed to directly control the intensity of an LCD backlight.
Various prior proposals have addressed the possible failure of display circuits. U.S. Pat. No. 5,559,528 discloses LCD and LED display circuits which employ redundant segments in order that segment failures are readily visually apparent.
U.S. Pat. No. 5,515,390 discloses an error detection apparatus for an LCD or any form of electro-optic display whose elements are capacitive, such as an electro-luminescent display. A comparator compares a voltage which is across the drive electrodes of the LCD element under test with a reference voltage. A resistor is connected across the drive electrodes to allow a controlled discharge of the parasitic capacitance under open circuit conditions of the element. When an incorrect electrode potential is detected, a control circuit generates an error warning, which can automatically prevent further operation (e.g., in a fuel dispenser application) until the fault is rectified. In this manner, an open circuit failure in the connections to, or within, the drive electrodes of the LCD, is detected.
U.S. Pat. No. 4,654,629 discloses a vehicle marker light using an array of LEDs in combination with lenses, such as Fresnel lenses, to provide a light beam of required intensity, shape and color for railroad applications. Voltage sensing circuitry, which includes a comparator and a reference voltage, senses the failure of the LEDs in the array and provides an indication of the failure.
In the railroad industry, "vital" is a term applied to a product or system that performs a function that is critical to safety, while "non-vital" is a term applied to a product or system that performs a function that is not critical to safety. Additionally, "fail-safe" is a design principle in which the objective is to eliminate the hazardous effects of hardware or software faults, usually by ensuring that the product or the system reverts to a state known to be safe.
For example, one of the components in the railroad industry which is included within an Automatic Train Protection (ATP) system is an Aspect Display Unit (ADU). The ATP receives signals from the rails which indicate the current maximum allowable speed for the locomotive. The ATP may then indicate this speed to the operator of the locomotive by controlling lights, called "aspects," on the ADU which is typically located between the two front windows or in the dash of the locomotive.
In many systems, the ADU is a non-vital element (i.e., not safety critical), since the ATP keeps the locomotive operating at or below the maximum allowable speed at all times by removing pressure from the brake system, thereby applying emergency brakes. In some systems, however, the ADU is a vital element since the ability of the system to control the brake system is either not available or can be selectively bypassed.
The "aspects" are normally light bulbs or blocks of LEDs which can be controlled using "vital outputs" whose failure modes result in removing voltage from across the LEDs. Conceptually, removing voltage from across the LEDs is easy to do since removing power ensures that the aspects will be dark, which has the same meaning as a zero mile-per-hour (MPH) allowable speed indication. Additionally, if the light bulbs or LEDs fail or burn out, then the aspect will go dark, thereby, providing a safe zero MPH allowable speed indication.
A problem with a seven-segment LED display results if one segment fails or is turned off inadvertently. Hence, the displayed allowable speed could actually increase in value. For example, if 80 MPH is the intended speed to be displayed with two seven-segment displays, and if the left bottom element (i.e., segment 7 of FIG. 1) in the seven-segment display for the "8" fails or is turned off, then the seven-segment display would show a "9" instead of an "8" and the speed shown would become 90 MPH instead of the intended 80 MPH. In other words, if a segment or its connection is damaged, then a wrong number could be displayed. As a further example, an "8" could be displayed as a "6" or a "0". For this reason, the fact that the output driving a particular LED is in the "on" state does not necessarily indicate that the LED is emitting light and showing a correct display.
U.S. Pat. No. 5,812,102 discloses two phototransistors as light sensors. The first phototransistor measures light transmitted from selectably energizable LEDs used to create a seven-segment display. The second phototransistor attempts to measure ambient light transmitted from external sources. Each segment of the display is surrounded by an opaque light shield to eliminate light transmissions from other segments in the ADU. A seven-segment display includes seven segments having seven primary photo-transistors and seven ambient photo-transistors enclosed within seven independent defined areas by seven shields, respectively. Because two phototransistors are enclosed in a transparent case, light refracted from the back of the segment can reach the phototransistors. In addition, some light reflected by the light shield reaches the second phototransistor. These effects reduce the sensitivity of the detection circuit. Accordingly, there is room for improvement.